s=input()

a=[s[0], s[1], s[2], s[3], s[0:2], s[1:3], s[2:4], s[0:3], s[1:4], s]
for j in range(len(a)):
    n=int(a[j])
    c=0
   
    for i in range(1, n+1):
        if n%i==0:
            c=c+1
    if c==2:
        mx=max(mx,n)
    
        


print(mx)
